This invention is directed to processes for upgrading the fraction boiling in the middle distillate range which is obtained from VGO hydrotreaters or moderate severity hydrocrackers. This invention involves a multiple-stage process employing a single hydrogen loop.
In the refining of crude oil, vacuum gas oil hydrotreaters and hydrocrackers are used to remove impurities such as sulfur, nitrogen, and metals from the crude oil. Typically, the middle distillate boiling material (boiling in the range from 250xc2x0 F.-735xc2x0 F.) from VGO hydrotreating or moderate severity hydrocrackers does not meet the smoke point, the cetane number or the aromatic specification. In most cases, this middle distillate is separately upgraded by a middle distillate hydrotreater or, alternatively, the middle distillate is blended into the general fuel oil pool or used as home heating oil. There are also streams in the diesel boiling range, from other units such as Fluid Catalytic Cracking, Delayed Coking and Visbreaking that require upgrading. Very often, existing diesel hydrotreaters are not designed to the pressure limits required to process these streams and the mild hydrocracking unit provides an opportunity for simultaneous upgrading of these streams.
There have been some previously disclosed processes in which hydroprocessing occurs within a single hydroprocessing loop. International Publication No. WO 97/38066 (PCT/US97/04270), published Oct. 16, 1997, discloses a process for reverse staging in hydroprocessing reactor systems. This hydroprocessor reactor system comprises two reactor zones, one on top of the other, in a single reaction loop. In the preferred embodiment, a hydrocarbon feed is passed to a denitrification and desulfurization zone, which is the lower zone. The effluent of this zone is cooled and the gases are separated from it. The liquid product is then passed to the upper zone, where hydrocracking or hydrotreating may occur. Deeper treating preferably occurs in the upper zone.
U.S. Pat. No. 5,980,729 discloses a configuration similar to that of WO 97/38066. A hot stripper is positioned downstream from the denitrification/desulfurization zone, however. Following this stripper is an additional hydrotreater. There is also a post-treat reaction zone downstream of the denitrification/desulfurization zone in order to saturate aromatic compounds. U.S. Pat. No. 6,106,694 discloses a similar configuration to that of U.S. Pat. No. 5,980,729, but without the hydrotreater following the stripper and the post-treat reaction zone.
With this invention, the middle distillate is hydrotreated in the same high pressure loop as the vacuum gas oil hydrotreating reactor or the moderate severity hydrocracking reactor, but the reverse staging configuration employed in the references is not employed in the instant invention. The investment cost saving and/or utilities saving involved in the use of a single hydrogen loop are significant since a separate middle distillate hydrotreater is not required. Other advantages include optimal hydrogen pressures for each step, as well as optimal hydrogen consumption and usage for each product. There is also a maximum yield of upgraded product, without the use of recycle liquid. The invention is summarized below.
A method for hydroprocessing a hydrocarbon feedstock, said method employing at least two reaction zones within a single reaction loop, comprising the following steps:
(a) passing a hydrocarbonaceous feedstock to a first hydroprocessing zone having one or more beds containing hydroprocessing catalyst, the hydroprocessing zone being maintained at hydroprocessing conditions, wherein the feedstock is contacted with catalyst and hydrogen;
(b) passing the effluent of step (a) directly to a hot high pressure separator, wherein the effluent is contacted with a hot, hydrogen-rich stripping gas to produce a vapor stream comprising hydrogen, hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrogen sulfide and ammonia and a liquid stream comprising hydrocarbonaceous compounds boiling approximately in the range of said hydrocarbonaceous feedstock;
(c) passing the vapor stream of step (b), after cooling and partial condensation, to a hot hydrogen stripper containing at least one bed of hydrotreating catalyst, where it is contacted countercurrently with hydrogen, while the liquid stream of step (b) is passed to fractionation;
(d) passing the overhead vapor stream from the hot hydrogen stripper of step (c), after cooling and contacting with water, the overhead vapor stream comprising hydrogen, ammonia, and hydrogen sulfide, along with light gases and naphtha to a cold high pressure separator, where hydrogen, hydrogen sulfide and light hydrocarbonaceous gases are removed overhead, ammonia is removed from the cold high pressure separator as ammonium bisulfide in the sour water stripper, and naphtha and middle distillates are passed to fractionation;
(e) passing the liquid stream from the hot hydrogen stripper of step (c) to a second hydroprocessing zone, the second hydroprocessing zone containing at least one bed of hydroprocessing catalyst suitable for aromatic saturation and ring opening, wherein the liquid is contacted under hydroprocessing conditions with the hydroprocessing catalyst, in the presence of hydrogen;
(f) passing the overhead from the cold high pressure separator of step (d) to an absorber, where hydrogen sulfide is removed before hydrogen is compressed and recycled to hydroprocessing vessels within the loop; and
(g) passing the effluent of step (e) to the cold high pressure separator of step (d).